On the structure and principles of air-operated open valves and air-operated closed valves

For pneumatic control valves: The so-called air-operated open type (K type) means that the greater the air pressure supplied to the actuator, the greater the degree of opening of the valve; when there is no air supply, the valve closes, which is referred to as FC (Fail Close)； The pneumatic shut-off type (Type B) means that the greater the air pressure supplied to the actuator, the smaller the valve opening; when the air supply is lost, the valve opens fully, which is referred to as FO (Fail Open). The choice of control valve depends on the specific process conditions. FC — Adjusting valve closes in case of air supply failure ； FO – Adjusting valve opens in case of air supply failure ； FL – The position of the control valve remains unchanged in case of a gas supply interruption (position retention) ； FLC – The position of the control valve remains unchanged in the event of a gas supply interruption (position maintenance); the valve closes at low signal levels ； FLO – The valve position remains unchanged in the event of a gas supply interruption (position retention); the valve opens under low signal conditions.

In other words, the valves are all the same; it’s just the position of the actuators that differs

For air-operated valves, when there is no air supply to the cylinder or diaphragm head, the valve stem is held in place by a spring, keeping the valve element in the closed position; such valves remain in a closed state (FC) in the event of an air supply failure； A gas-locked valve, as opposed to a gas-open valve, keeps the valve core in the fully open position with the valve stem held by a spring when no air is supplied to the cylinder or diaphragm head. Such a valve remains in the fully open state (FO) in the event of a failure in the air supply. There may be other types as well, but the principle should be similar.

In a pilot-operated control valve, the flow area increases as the signal pressure increases； In contrast, the air-shut type sees its flow cross-sectional area decrease as the signal pressure increases. The choice of operation mode mainly involves deciding between air-open or air-shut. There are mainly three considerations: (1) From the perspective of process safety, the principle is that in the event of a disruption in signal pressure, the safety of the equipment or operators must be ensured. If it is less hazardous for the valve to remain in the open position when the signal is interrupted, a air-shut type should be selected; otherwise, an air-open type should be used. (2) Considering the properties of the medium, if it is a material that tends to crystallize, a gas-operated valve should be used to prevent blockages. The heat exchanger maintains the outlet temperature of the cold fluid by adjusting the flow rate of the heat carrier; if the temperature of the cold fluid is too high, coking or separation may occur, affecting operation or damaging the equipment, and in such cases an air-operated control valve should be used. (3) From the perspective of ensuring product quality and minimizing economic losses, efforts should be made to reduce raw material and energy consumption as much as possible in the event of an accident, while still maintaining product quality

For pneumatic control valves: Air-operated (K-type) valves mean that the greater the air pressure supplied to the actuator, the greater the degree of opening of the valve; when there is no air supply, the valve closes, which is referred to as FC (Fail Close)； Air-locked (Type B) valves: These are valves for which the greater the air pressure supplied to the actuator, the smaller the opening degree of the valve; when the air supply is lost, the valve opens fully, which is referred to as FO (Fail Open).

Control valves are divided into pneumatic, electric, and hydraulic control valves. In simple terms, in a pressure-operated design, the valve stem is pushed open by air pressure; when the air supply is interrupted, the valve closes completely ； The pneumatic type operates by using air pressure to lower the valve stem; when the gas supply is interrupted, the valve opens fully. The entire process relies on the compressed air provided by the air compressor to open and close the valves.

What was said on the 2nd floor was very clear and concise!